Project Abstract The microbial community of the skin influences the function and development of the immune system, thus potentially participating in the pathophysiology of several important diseases of the skin including atopic dermatitis. Since normal microbial communities exist in equilibrium with the host, we have sought to better understand the mechanisms by which the microbiome controls mammalian immune responses. We have discovered that it was wrong to assume that the cutaneous microbiome is separated from dermal cells by the epidermis. Strong evidence now shows that microbial communities are in equilibrium across the basement membrane zone and populate the dermal stroma. These observations demonstrate that our microbiome can directly interact with dermal immunocytes, thus positioning these bacteria in a way to control immune function. Furthermore, this shows how barrier functions of the skin can influence the interaction between the microbiome and immunocytes. This conclusion provides the basis of the general hypothesis of this project that skin barrier function abnormalities in atopic dermatitis result in the abnormal entry of bacteria into the dermis, and this lack of normal barrier function drives an immune response characteristic of AD. This animal study seeks to address 2 fundamental questions: 1) Is the entry of microbes into the dermis altered in atopic dermatitis? and 2) How do these changes in the entry and identity of the dermal microbiome alter skin immune function? The specific aims of this project are: Aim 1: Determine how the entry of the surface microbiome is altered in mouse models of AD. Aim 2: Evaluate how the entry of specific bacteria into the dermis alters immune responses. Aim 3: Test how bacterial entry into the dermis modifies susceptibility to infection in AD.